1. Field of the Invention
The present invention relates to a polycarbonate diol having an extremely high ratio of primary hydroxyl groups in all terminal groups. More particularly, the present invention is concerned with a polycarbonate diol having diol monomer units and carbonate monomer units, wherein the amount of at least one diol monomer unit selected from the group consisting of a 1,5-pentanediol unit and a 1,6-hexanediol unit is from 50 to 100% by mole, based on the total molar amount of the diol monomer units, and wherein the ratio of primary hydroxyl groups in all terminal groups of the polycarbonate diol is in an extremely high, specific range and, therefore, the ratio of secondary terminal hydroxyl groups is extremely low. The present invention is also concerned with a thermoplastic polyurethane obtained by copolymerizing the above-mentioned polycarbonate diol and an organic polyisocyanate.
When the polycarbonate diol of the present invention is used for producing a thermoplastic polyurethane, a polyester elastomer and the like, the desired polymerization reactions can proceed at high rate, as compared to the case of the use of the conventional polycarbonate diol. Further, the thermoplastic polyurethane of the present invention has remarkably excellent properties with respect to strength, elongation, impact resilience and low temperature properties, as compared to the properties of a thermoplastic polyurethane obtained using the conventional polycarbonate diol.
2. Prior Art
A polyurethane and a urethane-, ester- or amide-based thermoplastic elastomer are used in the art. The soft segments of the polyurethane and thermoplastic elastomer are composed of structural units formed from a polyester polyol and/or a polyether polyol, each of which has a hydroxyl group at each of the molecular terminals thereof (for example, U.S. Pat. Nos. 4,362,825 and 4,129,715). A polyester polyol, such as a polyadipate polyol, has poor hydrolysis resistance. Due to the poor hydrolysis resistance, for example, a polyurethane containing, as soft segments, structural units formed from a polyester polyol has a disadvantage in that cracks are likely to occur and mold is likely to grow on the surface of the polyurethane within a relatively short period of time. Therefore, the use of such a polyurethane is considerably limited. On the other hand, a polyurethane containing, as soft segments, structural units formed from a polyether polyol has good hydrolysis resistance. However, the polyurethane has a disadvantage in that it has poor resistance to light and oxidative degradation. The disadvantages of these polyurethanes are, respectively, attributed to the presence of ester groups in the polymer chain and the presence of ether groups in the polymer chain.
A polycarbonate polyol prepared from 1,6-hexanediol is sold as a polyol usable for forming soft segments which have excellent resistance to hydrolysis, light, oxidative degradation, heat and the like. This resistance is due to the fact that carbonate linkages in the polymer chain exhibit extremely high chemical stability.
In recent years, a thermoplastic polyurethane which is produced using, as a soft segment, a copolycarbonate diol prepared from a mixture of 1,6-hexanediol and 1,4-butanediol or 1,5-pentanediol is attracting attention because of its great advantages. (The above-mentioned copolycarbonate diol is disclosed in Examined Japanese Patent Application Publication No. Hei 5-029648 (corresponding to EP 302712 and U.S. Pat. Nos. 4,855,377 and 5,070,173) and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-25264; and the above-mentioned thermoplastic polyurethane is disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-51428 and Japanese Patent Publication No. 1985394 (corresponding to EP 302712 and U.S. Pat. Nos. 4,855,377 and 5,070,173).) Specifically, such thermoplastic polyurethane has advantages not only in that it has remarkably excellent properties with respect to flexibility, low temperature properties and elastic recovery, as well as the same excellent properties as mentioned above and as achieved by using, as a soft segment, a polycarbonate diol prepared from 1,6-hexanediol, but also in that the thermoplastic polyurethane can be easily spun to produce a polyurethane fiber.
A polycarbonate diol is produced by effecting a transesterification reaction between a diol monomer having two primary hydroxyl groups and an organic carbonate compound, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate or diphenyl carbonate, in the presence or absence of a transesterification catalyst.
The prior art document disclosing a copolycarbonate diol prepared from a mixture of 1,6-hexanediol and 1,4-butanediol (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-25264) describes that an aliphatic polycarbonate diol containing 60 to 90% by mole of recurring units formed from 1,4-butanediol, based on the total molar amount of the diol monomer units, exhibits a stable reactivity in a reaction thereof with a polyisocyanate for forming a polyurethane.
On the other hand, however, in the course of the studies of the present inventors, it was found that an aliphatic polycarbonate diol, obtained by the method of Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-029648, specifically an aliphatic polycarbonate diol containing 50% by mole or more of a recurring unit selected from the group consisting of a 1,5-pentanediol unit and a 1,6-hexanediol unit, based on the total molar amount of the diol monomer units, has the following problems. When a polyurethane-forming reaction is conducted using such an aliphatic polycarbonate diol, not only does the polymerization reaction rate become low, but also the resultant thermoplastic polyurethane has poor properties with respect to strength, elongation, impact resilience, low temperature properties and the like. Further, when the above-mentioned aliphatic polycarbonate diol is used for the production of a polyester elastomer, the polymerization reaction rate becomes low.